This proposal concerns the function of the reciprocal loop connecting the visual claustrum and the visual cortex. Descriptions now exist of the response properties conveyed by the return limb of this loop, from claustrum to cortex, and its terminal distribution is also known. The most interesting question, that of the function of the cortico-claustral loop, remains to be answered. I intend to study this using three different methods. First, I will destroy the claustrum and assess the resulting changes in areas 18 and 19. This project is an extension of work carried out by Simon LeVay and myself on area 17, in which we demonstrated specific and reproducible alterations in the response properties of some cells following loss of claustral input to the cortex. Second, I will reversibly inactivate the claustrum using lidocaine and examine the consequences of loss of claustral input for single cells. In such experiments each cell will serve as its own control, rendering the method considerably more sensitive than ones dependent on irreversible lesions. Cells will be studied in areas 18 and 19, and then, depending on results in these areas, in areas 17 or PMLS, or both. Third, I will ask whether claustral axons branch to innervate more than one cortical area. To do so I will use two retrograde tracers in single experiments. One of these will be horseradish peroxidase, and the other, a tracer visualized autoradiographically. The discovery of the claustrum's function in visual cortical information processing should contribute significantly to our understanding of the generation of cortical response properties, a topic that had produced considerable interest but few definite answers. Moreover, this information might suggest what role the claustrum plays in other sensory modalities.